Murray.pa@XEROX.COM (01/09/87)
From Rudy: I have yet to hear a solid definition that is not disputed by someone for the terms "gateway", "router", "repeater" and "bridge". It's mailbox housecleaning time. I don't remember seeing a response to this corner of Rudy's message, so here is my 2 cents worth. I believe that the standards groups (ISO, ANSI, IEEE...) have converged upon the following meanings. This is an informal description, and slanted towards ethernets, but the translation to other hardware should be simple. In order of increasing complexity: Repeater: A box that copies each bit of a packet from one segment of a network to another. Thus if you put a scope on several chunks of coax connected by repeaters, you would observe the same packet at the "same" time if you are willing to ingore timing jitter up to several bit times. (On an ethernet, a repeater also has to copy the collision signal in the reverse direction.) It's possible to pry a repeater into 2 halves and connect them somehow, say with a pair of fibers. This gives you wider goegraphical coverage. The ethernet specs, for example, left room in their timing budget for 1000 meters of fiber inside the repeaters. Bridge: A box that selectivly copies raw packets from one one network to another network of the same type. This is reasonable to do, at least in theory, on an ethernet because the first few bytes of a raw ethernet packet contain the destination address, so a bridge that's watching all the traffic on an ethernet can deduce "Aah, host 12345657 is sending packets on segment 13, so it must be connected there and I should copy packets for it from other segments over to segment 13" and add that entry to its tables. There are several interesting ideas in bridges. First, the copy is selective so traffic on each section of the network is reduced from what would happen if you used repeaters rather than bridges. Second, raw packets get copied so this scheme works for all higher lever protocols, even ones that haven't been invented yet. Third, since whole packets get copied, the geographical or timing constraints of the basic physical network can be extended. Bridges can be split just like repeaters. Because of the filtering action, it might make sense to connect a pair of ethernets (10 megabits per second) with a (slower) T1 line (1.5 megabits per second). In theory, connections like this are "invisible" to the software on all the machines on the network so everything should just works fine, and probably does when things are lightly loaded. This isn't the place to go into the complications. Dave Mills has already contributed several war storys to TCP-IP. Router: A box that forwards packets of a particular protocol type (eg IP) from one logical network to another. The networks may be be of different physical types - eg ethernet to ring or arpanet to a phone line. (They can be the same - eg ethernet to another ethernet on a different floor.) In order to forward a packet, a router looks inside the packet to find the destination address, then consults its routing table which is normaly kept up to date by having routers talk to eachother via some routing protocol. Note that it's quite reasonable to have a multilingual router - a single machine that forwards packets for several protocol types - eg IP, Pup, and Chaos. Gateway: A box that translates from one protocol family to another, for example from Asyc RS232 connected to a dumb terminal to IP/TCP/Telnet, or from IP/TCP/SMTP mail to (Xerox) Pup/Grapevine mail. I think that "gateway" is the word responsible for most of the confusion. The Arpanet community (as well as the research corner of Xerox) uses "gateway" to mean what much of the rest of the world would call a "router". When I am being careful, I say "router", "packet gateway" or "mail gateway" in hopes of getting the correct binding. "Bridge" also contributes some confusion since several RFCs and messages refer to "mail bridges". I usually ignore that aspect and don't get burned very often. Another way to look at this mess is "What's a network?" In the ethernet world, a physical segment is a single length of coax. A physical network is several segments connected by repeaters. A logical network makes sense only relative to a particular protocol, and has a single network number. Logical networks have network numbers. Usually there is a one-to-one mapping between physical network and logical network, but things get blurred by subnets and bridges. An internet is a collection of logical networks (all speaking the same protocol) connected via routers. We also need a term for a clump of internets connected by, for example, mail gateways. I'd probably call it a metanet, but I don't think that term is in common usage. (If I get enough/any corrections, I'll collect and summarize.)
gross@MITRE-GATEWAY.ARPA.UUCP (01/09/87)
> We also need a term for a clump of internets connected by, for example, > mail gateways. I'd probably call it a metanet, but I don't think that > term is in common usage. When discussing a (future) large conglomeration of DoD and ISO nets that might include application level gateways, dual protocol gateways, dual protocol hosts and some form of interoperable DoD/ISO routing, we came up with the term "Teranet". (The greek prefix "tera-" means "so large and malformed as to be monsterous"; as in "teratology- the study of serious deviations from the normal type in organisms".)